Head expanders have been used with vibrational testing equipment such as shakers to substantially increase the table or plate area to which test specimens can be attached. Head expanders are desirable when the mass of the items being tested combined with the mass of the head expander is not sufficient to strain an existing shaker but the volume of the items is such that they cannot be accommodated on the head originally provided with the shaker.
Head expanders are known which enlarge a 24 inch diameter head to 36 or 40 inches. Such heads are driven by an armature which would typically provide 40,000 pounds of force in an up and down shaking frequency of up to 2,000 cycles per second. Unfortunately, expander heads under the above-mentioned shaking conditions heretofore have tended to diaphragm so that standing waves appear in their surfaces at critical frequencies. An object at the crest of such a standing wave is subjected to much more shaking force than one located at a node. Thus, test specimens positioned at different diameters or portions of the table are not subjected to similar amplitudes or displacements. Such expanders must be constructed as lightly as possible so sufficient vibrational energy remains for the mass of the items to be tested. Therefore, rather than being solid, they are constructed with a relatively thin table plate supported to the shaker head by supporting ribs. In an attempt to reduce diaphragming, some prior art head expanders include damping material, typically solid plastic material, with foam thereabout between the ribs underneath the table plate. The object being that the plastic material damps the standing waves. However, such arrangements are not particularly effective due to the magnitude of energy that must be damped especially at the outer portions of the table.